(1) Field of the Invention
The present invention relates to a modifier for modifying paving asphalt and a paving asphalt composition. More particularly, the present invention relates to an asphalt modifier which can easily be handled at the step of mixing the modifier into asphalt and will exert a highly improved modifying effect. The invention also relates to a paving asphalt composition containing this modifier.
(2) Description of the Prior Art
As one conventional method for preparing a paving asphalt composition having an improved flow resistance, there can be mentioned a method in which 100 parts of paving petroleum asphalt having a low softening point (about 50.degree. C.) is mixed with 25 to 100 parts of Trinidad Epure having a higher softening point (about 90.degree. to about 100.degree. C.), whereby deformation of an asphalt-paved road, which is readily caused at a high temperature, for example, in summer, is prevented. By the term "Trinidad Epure used herein, is meant a product obtained by purifying Trinidad lake asphalt, a kind of natural asphalt, which comprises 53 to 55% by weight of hard natural bitumen, 36 to 37% by weight of mineral substances, and 9 to 10% by weight of other substances.
In the method presently used with Trinidad Epure as a modifier for modifying paving asphalt, there can be mentioned a method in which Trinidad Epure is liquefied and is then added to asphalt and a method in which Trinidad Epure is added in the solid state.
When Trinidad Epure is used in the liquefied state, Trinidad Epure taken out from a storage vessel is roughly broken and heated and melted in a kettle equipped with a stirrer. Then the melted Trinidad Epure is mixed with paving petroleum asphalt after increasing the flowability of the melt by incorporation of an appropriate amount of paving petroleum asphalt, or the melt and paving petroleum asphalt are independently charged into a mixer for preparing the asphalt composition.
However, this method is defective in that long periods and a great deal of labor are necessary for pulverizing, heating and melting Trinidad Epure, and because mineral substances contained in Trinidad Epure are precipitated at the melting step, special care should be taken so as to prevent precipitation of these mineral substances at the melting and stirring step. Furthermore, areas where Trinidad Epure can be used is restricted.
A method for using Trinidad Epure in the solid state has been adopted in Germany. According to this method, Trinidad Epure stored in the pulverized state is directly charged into a mixer for preparing an asphalt composition, or is added into a mastic-preparing cooker of a lorry equipped with a stirrer. This method is economically advantageous over methods using Trinidad Epure in the liquefied state because the time is shortened and labor is saved.
When pulverized Trinidad Epure is used in the solid state, it readily undergoes re-caking during storage, and it is necessary to prevent this re-caking. Conventional pulverized Trinidad Epure, which has been subjected to re-caking-preventing treatment, include Trinidad Pulver and Trinidad Epuree Z. The former product comprises 50% by weight of finely divided Trinidad Epure and 50% by weight of paving stone powder, and the latter product comprises Trinidad Epure pulverized to a size of 12 to 0 mm and 3% by weight of diatomaceous earth, which is packed in a plastic bag.
In the case of trinidad Pulver, the bitumen content is as low as about 27% by weight, and transportation of Trinidad Pulver as the modifying bitumen is economically disadvantageous because stone powder should be transported. Furthermore, re-caking is caused at a high temperature according to the size of granulated Trinidad Epure. In case of Trinidad Epure Z, the bitumen content is satisfactorily high. However the content of diatomaceous earth is low, and if the absolute amount of diatomaceous earth is insufficient, and the average temperature is considerably higher than the method used in Germany, re-caking occurs and handling is difficult. Moreover, long-time storage is impossible. Therefore, both Trinidad Pulver or Trinidad Epure Z are insufficient as asphalt modifiers.